A blackbody absorbs all the radiation that falls on it, converts it into internal energy (heat), and then
re-radiates this energy into the surroundings. The re-radiated thermal energy,
known as blackbody radiation, has a continuous
spectrum governed solely by the body's temperature.

For any given temperature, there is a specific wavelength at which radiation emission is greatest (see diagram labeled "Variation
in blackbody curves with temperature").

The effective temperature (Te), or blackbody
temperature, is the surface temperature that an object, such as
a star, would have if it were a blackbody that radiated the same amount
of energy per unit area. This is a useful and widely employed measure of
stellar surface temperature. Te can be calculated from
the Stefan-Boltzmann law, which states that the total energy
radiated by a blackbody varies as the fourth power of its absolute temperature.
This law leads to the formula:

L = 4σR2Te4

where L is the luminosity of the
body, R is its radius, and σ (= 5.67 × 10-8 W/m2/K4) is the Stefan-Boltzmann constant. (A simplified
form appears in the box on blackbody radiation laws in the bottom illustration.)

Substituting solar values for L and R gives a value for the
effective temperature of the Sun of about 5,780 K.